At present, three methods are used extensively for the commercial application of printing ink to paper, i.e., offset, letterpress (and flexography), and gravure type printing. In offset printing, the printing plate has hydrophilic or "water-loving" non-printing areas and hydrophobic or "water-hating" printing areas, and is "planographic", i.e., the hydrophilic and hydrophobic areas of the plate all lie in the same plane, so that the plate has no relief. During printing, the printing plate does not come in direct contact with the paper to be printed, but rather transfers the inked image to a rubber blanket, which then applies the image to paper. Therefore, the process is an indirect, or offset method, and, hence, the name offset printing. The offset printing plate is initially wetted with an aqueous-based fountain solution that is preferentially adsorbed by the hydrophilic portion of the plate, and rejected by the hydrophobic portion. The plate is then contacted with a rubber roller laden with printing ink, which is rejected by the hydrophilic regions of the plate and accepted by the hydrophobic regions.
A significant advantage of offset printing is the ability to adequately print on relatively rough paper, due to the use of the rubber printing blanket, which is compressible, and, thus, allows intimate contact between the printing ink and the surface of the paper. Commonly used pigments in the coatings of paper used in offset printing include calcium carbonate and clay.
Letterpress and flexography are relief printing methods, in which the inked image portion of the plate is raised compared to the surrounding non-inked portion of the plate. Letterpress is typically a direct printing method, where the plate comes in direct contact with the paper. The high cost of the engravings required to produce the letterpress printing plate is a serious limitation of this process.
Gravure printing is an intaglio method, in which the image area contains recessed cells that are etched into a metallic printing plate to hold the printing ink. Ink is applied to the plate, filling the cells, where the amount of ink contained in each cell is determined by the depth of the cell. After the ink is applied to the gravure plate, the plate is wiped by a doctor blade that removes ink from the smooth, flat, non-image areas. In the most common form of gravure printing, a continuous roll or web of paper is printed, hence the name rotogravure. Although the cost of preparing a gravure printing plate or cylinder is much higher than preparing an offset printing plate, a rotogravure print run is typically very long, which offsets the cost of producing the gravure printing plate.
Since rotogravure is a direct printing method, the best results are obtained when the paper readily drains ink from the recessed cells without an excessive amount of pressure between the paper and the printing plate. Therefore, to obtain an acceptable rotogravure print, paper with the proper ink adsorption properties and good smoothness is required. A smooth and compressible paper is required to provide proper contact between the paper surface and the cell, so that each gravure cell in the printing plate is properly drained. Where contact between the paper surface and a cell is poor, the cell is not properly drained, resulting in a problem known as "missing dots". Therefore, the need for very smooth paper is a major limitation of rotogravure printing.
Good smoothness is typically achieved in paper for rotogravure printing by what is known in the art as "fiber coverage". In North American rotogravure formulations, fiber coverage is generally obtained by the use of large, platy clays, such as delaminated clay, to form a structure that bridges fibers in the paper. In Europe, talc is commonly used as an effective bridging pigment. Fiber coverage can also be enhanced with the use of structuring pigments, such as calcined clay, which improve the bulk of the coating, and can also improve compressibility.
Carbonates have rarely been used in rotogravure papers for a number of reasons, including high missing dots, increased roughness, and low gloss. In addition, most LWC rotogravure papers are still made by an acid papermaking process, and the carbonate, when used as a filler, will decompose in the acidic media used in the acid papermaking process. As a result, the acid papermaking process can tolerate only very small quantities of carbonate without experiencing serious processing problems. Therefore, even though calcium carbonate may be used as a coating pigment, where exposure to acidic conditions is limited, the teaching of the prior art has generally been that carbonates should not be used in rotogravure papers.